The following checklist is a starter list, ideal for triathletes new to the sport who are most likely participating in a pool swim event and who are likely using their normal road bike for the cycle.
Entry Details
Sponsorship Form/Money
 Often provides directions and organisation details, notification of start time and number and may occasionally be required for registration.
Trunks
Cycling/running shorts
Socks
Trainers (Cycling shoes)
Vest/Tshirt
 A good time saving tip is to wear the trunks/cycling style shorts for the whole event......you will soon warm up on the bike.
Talc in the shoes can aid easy transition without having to dry feet thoroughly and can save you getting blisters.
Warm top (ski stuff)
Warm leggings/tracky bottoms
Weatherproof over jacket
 You want something to keep warm in whilst you do your "warm up" and some layers for warming down after. Early and late season in this country is likely to be pretty nippy.
Bin Bag An alternative to the above if you have to lock away your other kit before the start. Can also double as protection against the elements in the transition area for your kit.
Flip Flops/sandals
Swimming Goggles
Swimming Cap
 If you want some footwear around the pool.............the swimming cap is often provided by the organisers but it is worth having a spare. Check about entry number on cap though!
Towel Best left in transition with other kit.
Heart Rate Strap & Monitor
 Although a tendency to being a nuissance in the swim it can be worn around the waste (under trunks) and "hitched up" into position after leaving the pool.
Bike Helmet
Gloves
Bike Pump
Spare Inner Tube
Saddle Bag + tools
 The helmet is one of the most essential items..........it is usually NO HELMET, NO RACE!!!!
Bike Lock Depends where you keep your bike before and after the race.
Sunglasses Okay so we're being optimistic............but a fly in the eye on the bike is a real pain and potentially disastrous! It is a good idea not to leave a good pair on display in transition - even at a triathlon.
Safety Pins 8 off, as numbers are often required on front and back of top. These may be provided by organisers but best to be sure.
Talcum Powder Helps with the slipping on of socks/trainers with wet feet.
Energy Bar(s)
Fruit
 You may not want to eat too much before hand but are likely to want something after.
Drink - Bike container (500ml min)
Drink before (500ml), & after Race (500ml) - all high Carb
 Make sure you keep hydrated before, during and after. Even a sprint distance event will require an above average intake.